Using high resolution solar X-ray spectra to benchmark the MEKAL spectral
synthesis code
K.J.H. Phillips
Space Science Department, CLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon.
OX11 0QX, U.K.
R. Mewe
SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
L.K. Harra-Murnion
Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, U.K.
J. Kaastra
SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
ABSTRACT
With the upcoming launch of XMM with its high resolution spectrometer (RGS)
onboard, it seems timely to review the accuracy of the spectral codes which will be
used extensively. In this work, high resolution solar spectra have been used as a
benchmark for the synthetic spectra derived from the MEKAL code. Improvements to
the code have been implemented.
1. Introduction
The Reection Grating Spectrometer (RGS) onboard the X-ray Multi-Mirror (XMM)
observatory will provide high resolution spectroscopy ( 0.05 A) in the wavelength range 5-35
A. XMM will provide observations of a large range of non-solar sources. A number of sources like
stellar coronae, clusters of galaxies and hot gas in interstellar and intergalactic medium are said
to be in coronal ionization equilibrium.
As much of the line emission data, in particular the line wavelengths, are derived from atomic
physics calculations, an observational benchmark, in which high-resolution spectra are compared
with MEKAL synthesized spectra, is clearly desirable. Since a number of the sources to be
observed with RGS are said to be in coronal ionization equilibrium, we can make use of high
resolution solar spectra to benchmark the spectral synthesis codes.
Very high-resolution soft X-ray spectra have been obtained in the past using crystal
spectrometers dedicated to solar active regions and ares. Among these are spectra from the
Flat Crystal Spectrometer (FCS), part of the X-ray Polychromator which was on board the Solar
{2{
Maximum Mission spacecraft (SMM), which operated between 1980 and 1989. This paper is
concerned with a benchmark study of the MEKAL code using solar are spectra from the FCS
instrument. Adjustments to the atomic code will be discussed in later sections.
2. Solar Flare Spectra
The SMM spacecraft operated fully for nine months in 1980, from the time of launch to the
time when an attitude control unit on the spacecraft failed, and from 1984 when Space Shuttle
astronauts repaired the attitude control unit to 1989 when the spacecraft re-entered the Earth's
atmosphere. The wavelength coverage of the Flat Crystal Spectrometer (FCS) was 1.5{20 A.
Despite operational problems, spectra taken on two occasions are suitable for analysis and
comparison with MEKAL theoretical spectra.
Flare 1 : Flare 1 was during the decay of an M3 are on August 25 1980. A single long
spectral scan covering the full range of the FCS was performed, starting at 13:10 U.T. and lasting
for approximately 20 minutes. There is signicant line emission in the range 5{20 A which is
covered by four of the seven FCS channels. A large number of the lines in the 10.5{17 A range are
due to Fe ions, with the majority due to 2{3 transitions in ions in the range Fexvii{Fexix.
Flare 2 : Several short spectral scans were made during a M4.5 are on July 2 1985 between
21:19 U.T. and 21:41 U.T., which included the peak at about 21:25 U.T. Subsequently, much
higher-excitation lines are apparent in these spectra, notably in the 7.8{10 A range which includes
intense 2{4 lines in various Fe ions (predominantly from Fexix to Fexxiv).
The accuracy of relative wavelengths given by (Phillips et al, 1982) is less than 0.002 A.
Absolute wavelengths were obtained using a reference line in each of the seven channels, generally
the resonance line (1s2 1 S0 , 1s2p 1 P0 ) of He-like ions, the theoretical wavelengths of which were
known to 0.001 A or less.
3. Spectral Synthesis by MEKAL
Analysis of the soft X-ray and EUV spectra from non-solar astronomical sources in the past
has generally made use of the several spectral synthesis codes that are available in the literature.
The MEKAL code, used via the SPEX spectral software package (Mewe et al, 1995, Kaastra et al,
1996), has been one of the most frequently used of these, and contains a considerable amount of
data relating to atomic transitions that give rise to both line and continuous spectra.
The original MEKA code (Kaastra & Mewe, 1993) was modied to allow for the fact that
comparison of theoretical spectra with those observed by the ASCA spacecraft of the central
regions of galaxy clusters (Fabian et al, 1994) showed a discrepancy with the intensity ratio of
{3{
the two groups of spectral lines due to 3{2 and 4{2 transitions in various ions of Fe (specically
Fexvii{Fexxiv). The addition of more than 2000 lines in the in the 7{19 A range using data from
the HULLAC (Hebrew University/Lawrence Livermore Atomic Code) code (Klapisch et al, 1977)
has enabled this discrepancy to be largely removed.
4. Reection Grating Spectrometer
XMM is designed specically to investigate in detail the X-ray emission characteristics of
cosmic sources. The wavelength band of the Reection Grating Spectrometer (RGS) is 5-35 A
(0.35-2.5 keV). It has a resolving power of 1000 which enables it to resolve the Fe L-shell
Fexvii{Fexxiv line complex (10-18 A), allowing more rened temperature diagnostics. The
He-like O VII triplet allowed density diagnostics and velocity diagnostics from various Fe lines
are permitted. Many of the lines in the Fe-L region are resolved emphasising the requirement for
accurate wavelengths in the synthetic spectra.
5. Corrections to the Wavelengths
We used the SMM spectra to benchmark the MEKAL wavelengths. The wavelengths of
lines which showed strong mis-matching were changed until the best 2 was achieved. The most
extreme examples are listed in Table 1. Figure 1 shows the new ts to the solar spectra. Figure 2
shows how important these changes are for the analysis of XMM RGS spectra.
Most of the lines are tted more accurately (e.g. the Mg XI forbidden line at 9.31 A, which is
blended with satellite lines j and k). This naturally produces a substantial change to the calulated
DEM.
Table 1: Examples of the corrections to the wavelengths
Ion
Fe XVIII
Fe XVII
Fe XVII
Fe XVII
Fe XVII
Fe XVII
Fe XVII
Fe XVII
A
11.761
12.134
12.274
13.795
15.272
16.796
17.071
17.119
orig
A
11.741
12.124
12.264
13.825
15.265
16.780
17.055
17.100
corr
6. Conclusions
With relatively small wavelength adjustments, we have been able to achieve very good
agreement between MEKAL and the solar are spectra. This is particularly true of the Fe L-line
{4{
Fig. 1.| Figure showing the SMM solar are spectra and the corrected t using the new
wavelengths
{5{
Fig. 2.| Figure showing the SMM solar are spectra and the corrected t using the new
wavelengths
{6{
complex, where several Fe ions have n = 2 , 3 transitions that fall in the 10{14 A (0.9{1.2 keV)
region and Fe n = 2 , 4 lines (observed in the hot July 2, 1985 are). When the comparison is
completed, we shall have a very complete line list in the X-ray spectral region 1:9 , 20 A and the
MEKAL code should give a very accurate representation of the X-ray spectra of sources in which
excitation conditions (electron collisional excitation, radiative decay) are like those in solar ares.
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This preprint was prepared with the AAS LATEX macros v4.0.